Arthroscopic devices and methods

ABSTRACT

An arthroscopic or other surgical cutter has features which facilitate fabrication by ceramic molding. The arthroscopic cutter includes a cutter body having a longitudinal axis and a window, an interior channel, and a plurality of cutting edges extending radially outwardly from an outer surface thereof. The features include non-helical, longitudinally aligned cutting edges, controlled thicknesses of the cutting edges, controlled heights of the cutting edges, controlled areas of the windows, controlled diameters of the internal channels, controlled rake angles of the cutting edges, and other parameters.

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of this application is related to that of applicationSer. No. 15/271,184, filed on the same day as this application.

BACKGROUND OF THE APPLICATION 1. Field of the Invention

This invention relates to arthroscopic tissue cutting and removaldevices by which anatomical tissues may be cut and removed from a jointor other site. More specifically, this invention relates to ceramiccutting members configured for use in arthroscopic cutters or shavers.

2. Description of the Background Art

In several surgical procedures including subacromial decompression,anterior cruciate ligament reconstruction involving notchplasty andarthroscopic resection of the acromioclavicular joint, there is a needfor cutting and removal of bone and soft tissue. Currently, surgeons usearthroscopic shavers and burrs having rotational cutting surfaces toremove tissue for such procedures. A typical arthroscopic shaver or burrcomprises a metal cutting member carried at the distal end of a metalsleeve that rotates within an open-ended metal shaft. A suction pathwayfor removal of bone fragments or other tissues is provided through awindow proximal to the metal cutting member that communicates with alumen in the sleeve.

When metal shavers and burrs ‘wear’ during a procedure, which occursvery rapidly when cutting bone, the wear can be characterized by loss ofmicro-particles from fracture and particle release which occurs alongwith dulling due to metal deformation. In such surgical applications,even very small amounts of such foreign particles that are not recoveredfrom a treatment site can lead to detrimental effects on the patienthealth, with inflammation being typical. In some cases, the foreignparticles can result in joint failure due to osteolysis, a term used todefine inflammation due to presence of such foreign particles. A recentarticle describing such foreign particle induced inflammation isPedowitz, et al. (2013) “Arthroscopic surgical tools: A source of metalparticles and possible joint damage”, Arthroscopy—The Journal ofArthroscopic and Related Surgery, 29(9), 1559-1565. In addition tocausing inflammation, the presence of metal particles in a joint orother treatment site can cause serious problems for future MRIs.Typically, the MRI images will be blurred by agitation of the metalparticles caused by the magnetic field used in the imaging, makingassessments of the treatment difficult.

Another problem with the currently available metal shavers/burrs relatesto manufacturing limitations in combination with the rapid dulling ofmetal cutting edges. Typically, a metal cutter is manufactured bymachining the cutting surfaces and flutes into a burr or abradersurface. The flute shape and geometry can be limited since it isdictated by the machining process, and burr size and shape limitationsmay direct usage toward more coarse bone removal applications. Further,when operated in a rotational or oscillatory mode, such cutting edgesadapted for coarse bone removal may have a kickback effect as the flutesfirst make contact with bone, which is aggravated by rapid dulling ofthe machined cutting edges.

Therefore, the need exists for arthroscopic burrs and/or shavers thatcan operate to cut and remove bone with minimal or no release offractured particles and micro-particles into the treatment site.Further, there is a need for burrs/cutters that do not wear rapidly andthat can have cutting edges not limited by metal machining techniques.Additionally, there is a need for such improved arthroscopic burrsand/or shavers with particular design features and parameters whichcontribute to achieving these design objectives. At least some of theseneeds will be met by the inventions described below.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a high-speed rotating cutter or cuttingmember that is fabricated entirely of a ceramic material, typically“comprising,” “consisting essentially of,” or preferably “consisting of”a single or monolithic ceramic structure configured to be fixedlymounted on a rotatable drive shaft of an arthroscopic or other surgicalcutting tool. As used herein after and in the claims, the phrase “cutterbody” will refer to the monolithic component of the cutter or cuttingmember which consists of the ceramic material, while the phrase “cuttingmember” will more broadly refer to the monolithic cutter body componentin combination with other metal or non-ceramic components whichoptionally form a portion of the cutter to be attached to a rotatabledrive shaft of an arthroscopic or other surgical cutting tool. In onevariation, the ceramic is a molded monolith with sharp cutting edges andis adapted to be motor driven at speeds of at least 1,000 RPM, typicallyranging from 15,000 RPM to 20,000 RPM. The ceramic cutting member iscoupled to an elongate inner sleeve that is configured to rotate withina metal, ceramic or composite outer sleeve. The ceramic material of thecutting member is exceptionally hard and durable and will not fractureand thus not leave foreign particles in a treatment site. In one aspect,the ceramic has a hardness of at least 8 Gpa (kg/mm²) and a fracturetoughness of at least 2 MPam^(1/2). The “hardness” value is measured ona Vickers scale and “fracture toughness” is measured in MPam^(1/2).Fracture toughness refers to a property which describes the ability of amaterial containing a flaw to resist further fracture and expresses amaterial's resistance to such fracture. In another aspect, it has beenfound that materials suitable for the cutting member of the inventionhave a certain hardness-to-fracture toughness ratio, which is a ratio ofat least 0.5 to 1.

While the cutting assembly and ceramic cutting member of the inventionhave been designed for arthroscopic procedures, such devices can befabricated in various cross-sections and lengths and can be use in otherprocedures for cutting bone, cartilage or soft tissue.

In a first aspect, the present invention provides an arthroscopic orother surgical cutter configured to be fabricated by ceramic molding.The arthroscopic cutter includes a cutter body having a longitudinalaxis and a plurality of cutting edges extending radially outwardly froman outer surface thereof. The cutter body is formed from awear-resistant ceramic material, and each cutting edge is non-helicaland aligned with the longitudinal axis to facilitate ceramic injectionmolding of the cutter body.

In specific embodiments, at least some of the non-helical cutting edgesof such arthroscopic cutters have a linear cutting edge and/or have asubstantially flat cutting face. In other embodiments, the cutter bodymay have an outer diameter measured at the periphery of the cuttingedges and a radial height of the cutting edges or cutting faces, whereina ratio of the radial height of the cutting faces to the outer diameterof the cutter body is 0.2:1 or less, often being 0.1:1 or less.

In a second aspect, the present invention provides an arthroscopiccutter including a ceramic cutter body with a plurality of cutting edgesseparated by flutes on an outer surface thereof. Each cutting edgetypically has a cutting face with a cutting face height measured from aninnermost peripheral location on a flute to an outermost peripherallocation on an adjacent cutting face and a cutting edge thicknessmeasured along a tangent to a midpoint of the cutting face to theadjacent flute. The ratio of the cutting edge thickness to the cuttingface height is at least 1.5:1.

In specific embodiments, the ratio of the cutting edge thickness to thecutting face height may be at least 2:1 and the ceramic body may be atleast one of yttria-stabilized zirconia, magnesia-stabilized zirconia,ceria-stabilized zirconia, zirconia toughened alumina or siliconnitride. Typically, the ceramic cutter body has a hardness of at least 8Gpa (kg/mm²) and a fracture toughness of at least 2 MPam^(1/2).

In a third aspect, the present invention provides a surgical device forcutting bone comprises a cutter body having a longitudinal axis and aplurality of cutting edges. The cutter body is fabricated of a wearresistant ceramic, and each cutting edge is aligned with saidlongitudinal axis. Each cutting edge also has a radial rake angleranging between −5° and +10°.

In a fourth aspect, the present invention provides an arthroscopiccutter comprising a ceramic cutter body with a plurality of cuttingedges and flutes intermediate the cutting edges. Each cutting edge has aland with a clearance of less than 5% of the outer cutting edge diameterat a radial angle of 15° behind the cutting edge.

In a fifth aspect, the present invention provides an arthroscopic cuttercomprising a cutter body fabricated of a wear resistant ceramicconfigured to be carried by an elongate shaft. The cutter body has aplurality of cutting edges separated by flutes, and each cutting edgehas a height measured from an innermost peripheral location on a fluteto an outermost peripheral location on an adjacent cutting edge. Thecutter body has an interior channel and a window with a width andlength, and the window is open to the interior channel, and the interiorchannel is configured to communicate with a passageway in the shaft. Theratio of window width to the cutting edge height is at least 5:1.

In a sixth aspect, the present invention provides an arthroscopic cuttercomprising a cutter body fabricated of a wear resistant ceramic. Thecutter body is configured to be carried by an elongate shaft and has awindow, a central channel, and a plurality of cutting edges separated byflutes. Each cutting edge defines a cutting face having a surface area,and the window has an area that is open to the central channel andwherein the central channel is configured to communicate with apassageway in the shaft. The ratio of window area to the cutting surfaceface area is at least 8:1.

In a seventh aspect, the present invention provides an arthroscopiccutter comprising a cutter body fabricated of a wear resistant ceramicand configured to be carried by an elongate shaft. The cutter body has awindow, an interior channel having a diameter, and a plurality ofcutting edges separated by flutes. Each cutting edge has a heightmeasured from an innermost peripheral location on a flute to anoutermost peripheral location on an adjacent cutting edge, and thewindow has an area that is open to the interior channel. The interiorchannel has a diameter and is configured to communicate with apassageway in the elongate shaft, and the ratio of the interior channeldiameter to the cutting edge height is at least 2:1, often being atleast 4:1, and sometimes being at least 6:1.

In an eighth aspect, the present invention provides an arthroscopiccutter configured for ceramic molding, comprising a cutter bodyfabricated of a wear resistant ceramic material and having alongitudinal axis, a distal portion having cutting edges, and a proximalshaft portion, and a window in the proximal shaft portion, wherein thewindow has lateral edges with a non-zero positive radial rake angle.

In specific embodiments, the lateral window edges of the arthroscopiccutter may have a radial rake angle ranging from 15° to 45°. In otherspecific embodiments, a distal end of the window may be axiallyspaced-apart from a proximal end of the cutting edges by less than 0.10inch, often by less than 0.05 inch.

The inventions of the present application are also defined and can alsobe understood with reference to the following numbered clauses:

1. An arthroscopic cutter, comprising: a ceramic cutter body having alongitudinal axis and a plurality of cutting edges separated by fluteson an outer surface thereof; wherein each cutting edge has a cuttingface with a radial cutting face height measured from an innermostperipheral location on a flute to an outermost peripheral location on anadjacent cutting face and a cutting edge thickness measured along atangent to a midpoint of the cutting face to the adjacent flute; andwherein the ratio of the cutting edge thickness to the cutting faceheight is at least 1.5:1.

2. The arthroscopic cutter of clause 1 wherein the ratio of the cuttingedge thickness to the cutting face height is at least 2:1.

3. The arthroscopic cutter of clause 1 wherein each cutting edge isnon-helical and aligned with the longitudinal axis.

4. The arthroscopic cutter of clause 1 wherein least some of the cuttingedges have a linear cutting edge.

5. The arthroscopic cutter of clause 1 wherein at least some of thecutting edges have a substantially flat cutting face.

6. The arthroscopic cutter of clause 1 wherein each cutting edge has aland with a clearance of less than 5% of the outer cutting edge diameterat a radial angle of 15° behind the cutting edge.

7. The arthroscopic cutter of clause 1 wherein the cutter body has anouter diameter through the longitudinal axis at the outermost peripheryof the cutting edges wherein a ratio of the cutting face height to theouter diameter of the cutter body is 0.2:1 or less.

8. The arthroscopic cutter of clause 7 wherein a ratio of the cuttingface height to the outer diameter of the cutter body is 0.1:1 or less.

9. The arthroscopic cutter of clause 1 wherein each cutting face has aradial rake angle ranging between −5° and +10°.

10. The arthroscopic cutter of clause 1 wherein the cutter body has aninterior channel and a window with a width and length, the window beingopen to the interior channel therein and configured to communicate witha passageway in a shaft that carries the cutter body, wherein the ratioof window width to the cutting face height is at least 5:1.

11. The arthroscopic cutter of clause 10 wherein each cutting face has asurface area and the window has an area that is open to the interiorchannel; and wherein the ratio of window area to the cutting facesurface area is at least 8:1.

12. The arthroscopic cutter of clause 10 wherein the interior channelhas a diameter and is configured to communicate with the passageway inthe shaft; and wherein the ratio of the interior channel diameter to thecutting face height is at least 2:1.

13. The arthroscopic cutter of clause 12 wherein the ratio of theinterior channel diameter to the cutting face height is at least 4:1.

14. The arthroscopic cutter of clause 12 wherein the ratio of theinterior channel diameter to the cutting face height is at least 6:1.

15. The arthroscopic cutter of clause 10 wherein the cutter body has adistal portion comprising the cutting edges and a proximal shaft portionhaving the window; and wherein the window has lateral edges with anon-zero positive radial rake angle.

16. The arthroscopic cutter of clause 15 wherein said lateral edges havea radial rake angle ranging from 15° to 45°.

17. The arthroscopic cutter of clause 15 wherein a distal end of thewindow is axially spaced apart from a proximal end of the cutting edgesby less than 0.10 inch.

18. The arthroscopic cutter of clause 15 wherein a distal end of thewindow is axially spaced apart from a proximal end of the cutting edgesby less than 0.05 inch.

19. An arthroscopic cutter configured to be fabricated by ceramicmolding, said arthroscopic cutter comprising: a cutter body having alongitudinal axis and a plurality of cutting edges separated by flutes,the cutting edges extending radially outwardly from an outer surfacethereof; wherein the cutter body is formed from a wear-resistant ceramicmaterial; and wherein each cutting edge is non-helical and aligned withthe longitudinal axis to facilitate ceramic injection molding of thecutter body.

20. The arthroscopic cutter of clause 19 wherein each cutting edge has acutting face with a radial cutting face height measured from aninnermost peripheral location on a flute to an outermost peripherallocation on an adjacent cutting face and a cutting edge thicknessmeasured along a tangent to a midpoint of the cutting face to theadjacent flute; and wherein the ratio of the cutting edge thickness tothe cutting face height is at least 1.5:1.

21. The arthroscopic cutter of clause 20 wherein the ratio of thecutting edge thickness to the cutting face height is at least 2:1.

22. The arthroscopic cutter of clause 20 wherein the cutter body has adistal portion comprising the cutting edges and a proximal shaft portionhaving a window open to the interior channel therein; and wherein thewindow has lateral edges with a non-zero positive radial rake angle.

23. The arthroscopic cutter of clause 22 wherein said lateral edges havea radial rake angle ranging from 15° to 45°.

24. The arthroscopic cutter of clause 22 wherein the window and interiorchannel are configured to communicate with a passageway in a shaft thatcarries the cutter body, wherein the ratio of the window width to thecutting face height is at least 5:1.

25. The arthroscopic cutter of clause 22 wherein each cutting face has asurface area and the window has an area that is open to the interiorchannel; and wherein the ratio of window area to the cutting facesurface area is at least 8:1.

26. The arthroscopic cutter of clause 22 wherein the interior channelhas a diameter and is configured to communicate with the passageway inthe shaft; and wherein the ratio of the interior channel diameter to thecutting face height is at least 2:1.

27. The arthroscopic cutter of clause 22 wherein the ratio of theinterior channel diameter to the cutting face height is at least 4:1.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be discussed withreference to the appended drawings. It should be appreciated that thedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting in scope.

FIG. 1 is a perspective view of a disposable arthroscopic cutter orshaver assembly with a ceramic cutting member carried at the distal endof a rotatable inner sleeve with a window in the ceramic cutting memberproximal to the distal cutting edges.

FIG. 2 is a perspective view of a handle body with a motor drive unit towhich the cutter of FIG. 1 can be coupled, with the handle bodyincluding an LCD screen for displaying operating parameters of deviceduring use together with a joystick and mode control actuators on thehandle.

FIG. 3 is an enlarged perspective view of the ceramic cutting member ofthe arthroscopic cutter or shaver assembly of FIG. 1.

FIG. 4A is an enlarged end view of the ceramic cutting member of FIG. 3corresponding to the invention showing several features including threenon-helical, axis-aligned cutting edges, the cutting edge height and thecutting edge thickness.

FIG. 4B is a sectional view of a hypothetical ceramic cutting memberwith a high cutting edge height and a low cutting edge thickness whichillustrates how the cutting edges would fracture.

FIG. 5A is a perspective view of a prior art metal burr with eightelongate cutting edges and an aspiration window positioned proximal tothe cutting edges.

FIG. 5B is a cross-sectional view of the prior art metal burr of FIG. 5Ataken along line 5B-5B of FIG. 5A showing the metal burr's radial rakeangle, cutting edge height, cutting edge thickness, primary clearanceangle and land width.

FIG. 6 is an end view of a portion of the ceramic cutting member ofFIGS. 3 and 4A showing the radial rake angle of the cutting edges and ameans for defining the thickness of the cutting edges.

FIG. 7A is a cut-away sectional schematic view of a multi-componentceramic injection mold illustrating a method of fabricating a ceramiccutting member (shown in a side view) with core pins configured to formthe window and the interior channel in the cutting member.

FIG. 7B another cut-away view of the multi-component ceramic injectionmold of FIG. 7A showing schematically the first and second core pinsafter being removed and a first mold component being moved in alignmentwith the axis of the cutting member to release the distal cutting edgeportion from the mold.

FIG. 7C another cut-away view of the multi-component ceramic injectionmold of FIGS. 7A-7B showing schematically second and third moldcomponents being moved away from the axis of the cutting member torelease the proximal shaft portion from the mold.

FIG. 8A is a cut-away view of a multi-cavity ceramic injection mold formolding a plurality of cutting members that shows cutting members in atop view, with the mold operating similar to the single cavity mold ofFIGS. 7A-7C.

FIG. 8B is another cut-away view of the multi-cavity mold of FIG. 8Ashowing a core ping being removed and a mold component being moved inalignment with the axis of the cutting member to release the distalcutting edge portion from the mold similar to that of FIG. 7B.

FIG. 9 is a side elevation view of the ceramic cutting member of FIGS. 3and 4A showing the dimension of the interior channel.

FIG. 10 is a sectional view of the cutting member of FIG. 9 taken alongline 10-10 of FIG. 9 showing another view of the interior channel andthe window having longitudinal edges that have a sharp apex and a highpositive radial rake angle.

FIG. 11 is a perspective view of the cutting member of FIGS. 3, 4A and 9schematically showing the cutting of bone chips and the resultingcross-section of such bone chips.

FIG. 12A is a sectional view of the shaft portion and window of thecutting member of FIGS. 3, 4A and 9 schematically rotating in relationto bone chips.

FIG. 12B is a sectional view of the cutting member window of FIG. 12Aafter further rotation wherein the window's sharp apex and a highpositive radial rake angle capture bone chips.

FIG. 13A is a sectional view of the window of the prior art metal burrof FIG. 5A taken along line 13A-13A of FIG. 5A schematically rotating inrelation to bone chips.

FIG. 13B is another view of the prior art metal burr window of FIG. 13Aafter further rotation wherein the window's negative radial rake angledoes not facilitate the capture bone chips.

FIG. 14A is a cut-away sectional schematic view of a single-cavityceramic injection mold illustrating a method of fabricating a ceramiccutting member with two cutting edges wherein the mold has only twoparting components with a parting line on the centerline of the cuttingmember.

FIG. 14B another cut-away view of the injection mold of FIG. 14A showingschematically a first mold component being moved away from the greencutting member.

FIG. 15A is a cut-away sectional schematic view of another ceramicinjection mold with only two parting components that is configured forfabricating a ceramic cutting member with three cutting edges whereinthe mold has a parting line that is off-center relative to the cuttingmember axis.

FIG. 15B another cut-away view of the injection mold of FIG. 15A showingschematically a first mold component being moved away from the cuttingmember axis which is enabled by flat side portions of the cutting memberwhich eliminates undercuts.

FIG. 15C another view of the injection mold of FIGS. 15A-15B showing thegreen cutting member being released from the mold component.

FIG. 16A is a cut-away view of another multi-cavity injection mold withsome feature similar to that of FIGS. 8A-8B except that the ceramiccutting member has helical cutting edges.

FIG. 16B shows schematically the method of helically moving moldcomponents to release the green ceramic cutting member from its helicalcutting edges.

FIG. 17 is a cross-sectional view of another mold similar to thosedescribed above showing a core pin with non-parallel sides that can beused to form window edges that various positive radial rake angles.

FIG. 18 is a cross-sectional view of another mold similar to thosedescribed above showing a core pin that extends through the ceramiccutting member to provide windows in both sides of the cutting member.

FIG. 19 is a longitudinal sectional view of another mold similar tothose described above showing a core pin that for forming a window thatis angled longitudinally relative to the axis of the cutting member.

FIG. 20 is a schematic view of another mold similar to those describedabove showing a ceramic cutting member with a window having non-parallelsides which can be formed by a core pin.

FIG. 21 is a perspective view of a cutting member similar to that ofFIGS. 3 and 4A showing an interior channel that includes an off-centerdistal portion to create a weight asymmetry in the distal portion of thecutting member to counter-balance the weight asymmetry in the shaftportion caused by the window.

FIG. 22 is a sectional view of another cutting member similar to that ofFIGS. 3 and 4A showing an off-center interior channel to create a weightasymmetry in the cutting member to counter-balance the weight asymmetryin the shaft portion caused by the window.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to bone and cutting and removal devicesand related methods of use. Specific variations of a ceramic cutter ofthe invention will be described to provide an overall understanding ofthe principles of the form and function as well as the methods of use ofthe devices disclosed herein.

In general, the present disclosure provides an arthroscopic cutter forcutting bone. The cutter is intended to be disposable and is configuredfor detachable coupling to a handle and motor drive component which isintended to be non-disposable. This description of the generalprinciples of these inventions below is not meant to limit the inventiveconcepts in the appended claims.

In general, the present invention provides a high-speed rotating ceramiccutter or “burr” that is configured for use in many arthroscopic andother surgical applications, including but not limited to treating bonein shoulders, knees, hips, wrists, ankles and the spine. Moreparticularly, devices of the present invention include a cutting memberthat is fabricated entirely of a ceramic material selected so that thecutting member is extremely hard and durable, as described in detailbelow. A motor drive is operatively coupled to the ceramic cutter orburr to rotate the burr edges at rotational speeds of at least 1,000RPM, typically ranging from 3,000 RPM to 20,000 RPM. As will bedescribed further below, in a particular variation, the ceramic cutteris operated at 16,500 RPM for cutting bone.

In one variation shown in FIGS. 1-2, an arthroscopic cutter or cutterassembly 100 is provided for cutting and removing hard tissue, whichoperates in a manner similar to commercially available metals shaversand burrs. FIG. 1 shows disposable cutter assembly 100 that is adaptedfor detachable coupling to a handle 104 and motor drive unit 105 thereinas shown in FIG. 2.

The cutter assembly 100 of FIG. 1 has a shaft 110 extending alonglongitudinal axis 115 that comprises an outer sleeve 120 and an innersleeve 122 rotatably disposed therein with the inner sleeve 122 carryinga distal ceramic cutting member 125 having an interior channel 126therein that communicates with lumen 128 in the inner sleeve 122. Theshaft 110 extends from a proximal hub assembly 132 wherein the outersleeve 120 is coupled in a fixed manner to an outer hub 140A which canbe an injection molded plastic, for example, with the outer sleeve 120insert molded therein. The inner sleeve 122 is coupled to an inner hub140B (phantom view) that is configured for coupling to the motor driveunit 105 (FIG. 2). The outer and inner sleeves 120 ands 122 typicallycan be a thin wall stainless steel tube, but other materials can be usedsuch as ceramics, metals, plastics or combinations thereof.

Referring to FIG. 1, the outer sleeve 120 extends to distal sleeveregion 142 that has an open end and cut-out 144 that is adapted toexpose a window 145 in the ceramic cutting member 125 during a portionof the inner sleeve's rotation. The window 145 communicates withinterior channel 126 in the cutting member 125. Referring to FIGS. 1 and2, the proximal hub 132 of the cutter assembly 100 is configured with aJ-lock, snap-fit feature, screw thread or other suitable feature fordetachably locking the hub assembly 132 into the handle 104. As can beseen in FIG. 1, the outer hub 140A includes a projecting key 146 that isadapted to mate with a receiving J-lock slot in the handle 104 (see FIG.2).

In FIG. 2, it can be seen that the handle 104 is operatively coupled byelectrical cable 152 to a controller 155 which controls the motor driveunit 105. Actuator buttons 156 a, 156 b or 156 c on the handle 104 canbe used to select operating modes, such as various rotational modes forthe ceramic cutting member. In one variation, a joystick 158 be movedforward and backward to adjust the rotational speed of the ceramiccutting member 125. The rotational speed of the cutter can continuouslyadjustable, or can be adjusted in increments up to 20,000 RPM. FIG. 2further shows that negative pressure source 160 is coupled to aspirationconnector 162 which communicates with a flow channel 164 in the handle104 and through the shaver hub 132 (FIG. 1) to the lumen 128 in innersleeve 122 which extends to window 145 in the ceramic cutting member 125(FIG. 2).

Now referring to FIGS. 3 and 4A, the cutting member 125 comprises aceramic body or monolith that is fabricated entirely of a technicalceramic material that has a very high hardness rating and a highfracture toughness rating, where “hardness” is measured on a Vickersscale and “fracture toughness” is measured in MPam^(1/2). Fracturetoughness refers to a property which describes the ability of a materialcontaining a flaw or crack to resist further fracture and expresses amaterial's resistance to brittle fracture. The occurrence of flaws isnot completely avoidable in the fabrication and processing of anycomponents. The inventors evaluated technical ceramic materials andtested prototypes to determine which ceramics are best suited for thenon-metal cutting member 125. When comparing the material hardness ofthe ceramic cutters of the invention to prior art metal cutters, it caneasily be understood why typical stainless steel bone burrs are notoptimal. Types 304 and 316 stainless steel have hardness ratings of 1.7and 2.1, respectively, which is low and a fracture toughness ratings of228 and 278, respectively, which is very high. Human bone has a hardnessrating of 0.8, so a stainless steel cutter is only about 2.5 timesharder than bone. The high fracture toughness of stainless steel resultsin ductile behavior which thus results in rapid cleaving and wear onsharp edges of a stainless steel cutting member. In contrast, technicalceramics materials have a hardness ranging from approximately 10 to 15,which is five to six times greater than stainless steel and which is 10to 15 times harder than cortical bone. As a result, the sharp cuttingedges of a ceramic remain sharp and will not become dull when cuttingbone. The fracture toughness of suitable ceramics ranges from about 5 to13 which is sufficient to prevent any fracturing or chipping of theceramic cutting edges. The inventors determined that ahardness-to-fracture toughness ratio (“hardness-toughness ratio”) is auseful term for characterizing ceramic materials that are suitable forthe invention as can be understood form the Chart A below, which listshardness and fracture toughness of cortical bone, a 304 stainless steel,and several technical ceramic materials.

CHART A Ratio Hardness Fracture to Hardness Toughness Fracture (GPa)(MPam^(1/2)) Toughness Cortical bone 0.8 12  .07:1 Stainless steel 3042.1 228  .01:1 Yttria-stabilized zirconia (YTZP) 12.5 10 1.25:1 YTZP2000 (Superior Technical Ceramics) YTZP 4000 (Superior Technical 12.5 101.25:1 Ceramics) YTZP (CoorsTek) 13.0 13 1.00:1 Magnesia stabilizedzirconia (MSZ) 12.0 11 1.09:1 Dura-Z ® (Superior Technical Ceramics) MSZ200 (CoorsTek) 11.7 12 0.98:1 Zirconia toughened alumina (ZTA) 14.0 52.80:1 YTA-14 (Superior Technical Ceramics) ZTA (CoorsTek) 14.8 6 2.47:1Ceria stabilized zirconia 11.7 12 0.98:1 CSZ (Superior TechnicalCeramics) Silicon Nitride 15.0 6 2.50:1 SiN (Superior TechnicalCeramics)

As can be seen in Chart A, the hardness-toughness ratio for the listedceramic materials ranges from 98× to 250× greater than thehardness-toughness ratio for stainless steel 304. In one aspect of theinvention, a ceramic cutter for cutting hard tissue is provided that hasa hardness-toughness ratio of at least 0.5:1, 0.8:1 or 1:1.

In one variation, the ceramic cutting member 125 of FIG. 3 is a form ofzirconia. Zirconia-based ceramics have been widely used in dentistry andsuch materials were derived from structural ceramics used in aerospaceand military armor. Such ceramics were modified to meet the additionalrequirements of biocompatibility and are doped with stabilizers toachieve high strength and fracture toughness. The types of ceramics usedin the current invention have been used in dental implants, andtechnical details of such zirconia-based ceramics can be found inVolpato, et al., “Application of Zirconia in Dentistry: Biological,Mechanical and Optical Considerations”, Chapter 17 in Advances inCeramics—Electric and Magnetic Ceramics, Bioceramics, Ceramics andEnvironment (2011).

In one variation, the ceramic cutting member 125 of FIG. 3 is fabricatedof an yttria-stabilized zirconia as is known in the field of technicalceramics, and can be provided by CoorsTek Inc., 16000 Table MountainPkwy., Golden, Colo. 80403 or Superior Technical Ceramics Corp., 600Industrial Park Rd., St. Albans City, Vt. 05478. Other technicalceramics that may be uses consist of magnesia-stabilized zirconia,ceria-stabilized zirconia, zirconia toughened alumina and siliconnitride. In general, in one aspect of the invention, the monolithicceramic cutting member 125 has a hardness rating of at least 8 Gpa(kg/mm²). In another aspect of the invention, the ceramic cutting member125 has a fracture toughness of at least 4 MPam^(1/2).

The fabrication of such ceramics or monoblock components are known inthe art of technical ceramics, but have not been used in the field ofarthroscopic cutting or resecting devices. Ceramic part fabricationincludes molding a part such as a cutting member 125 which is called“green” after release from a mold, then sintering or “firing” the moldedgreen part at high temperatures over precise time intervals to transformthe compressed ceramic powder into a ceramic monoblock which can providethe hardness range and fracture toughness range as described above.Injection molds for fabricating such ceramic cutting members aredescribed in more detail below.

In FIG. 3, one variation of ceramic cutter 125 is shown which has aproximal shaft portion 170 and distal portion 172 which has cuttingedges 175 extending radially outward from the outer surface OS of thecutting member. The shaft portion 170 has a reduced diameter section 180that includes projecting elements 182 which engage receiving openings inthe inner sleeve 122 for locking the cutting member 125 to the innersleeve 122 (phantom view). A thin-wall polymeric sleeve 184, forexample, heat shrink tubing is shown in phantom view in FIG. 3 extendingover the inner sleeve 122 and the reduced diameter section 180 shaftportion 170 to provide a lubricious, dielectric outer layer covering theinner sleeve 122. In other variations, a ceramic cutting member 125 canbe coupled to metal sleeve 122 by brazing, adhesives, threads or acombination thereof. Still referring to FIG. 3, the window 145 in theceramic cutting member 125 can extend over a radial angle rangingbetween about 15° to 90° of the shaft portion 170. In a variation, thewindow 145 is provided with sharp outer edges 185A and 185B with a highradial rake angle for capturing bone chips and cutting soft tissue aswill be described further below. Further, the bone chips or the resectedsoft tissue are moved or suctioned by the negative pressure source 160through the window 145 and interior channel 126 (with diameter C) in theceramic member 125 and thereafter into the increased diameter lumen 128of the inner sleeve 122 (see FIG. 1). The increase in diameter fromchannel 126 to inner sleeve lumen 128 is advantageous for providing aclog-free outflow pathway as any removed tissue that passes through theinterior channel 126 in the ceramic cutting member 125 will be thenentrained in fluid outflows in inner sleeve lumen 128.

As will be described next, a ceramic cutting member 125 of FIG. 3corresponding to the invention has many unique features for functionalpurposes that distinguish its shape and configuration from prior artmetal burrs or blades. After extensive testing, it has been found thatan optimized ceramic cutting member 125 differs from typical metal burrs(see FIGS. 5A-5B) in several ways, including (i) the number of cuttingedges, (ii) the height of the cutting edges, (iii) the thickness of thecutting edges (iv) the length and surface area of the cutting edges, and(v) the dimensions, configuration and location of the window in thecutting member. Further, the system of the invention uses higherrotational speeds than prior art systems for optimizing use of a ceramiccutter in cutting bone.

In the variation shown in FIGS. 3 and 4A, the ceramic cutting member orcutter body has three cutting edges 175 and three flutes 180 with theouter diameter or cutting edge periphery P being cylindrical and taperedor rounded in the distal direction. Metal shaver blades typically havesix, eight or more cutting edges. FIGS. 5A-5B show a prior art metalshaver blade or burr 186 with eight cutting edges 175′ and eightintermediate flutes 180′.

As can be seen in FIGS. 3 and 4A, the cutting edges 175 in ceramiccutting member 125 are typically non-helical, e.g., linear or straight,and aligned with longitudinal axis 115 to facilitate injection moldingas will be described below. FIG. 4A shows a cutting edge 175 as beingdefined as the hatched area that extends radially outward from the outersurface OS. In one aspect, it has been found that the ceramic cutter 125(FIGS. 3 and 4A) with fewer cutting edges 175 than a metal burr isoptimal for bone cutting. Further, an optimal ceramic cutter has cuttingedges with a height A which is much less than a cutting edge height in atypical metal burr (see FIGS. 5A-5B). The ceramic cutting member 125 inFIG. 4A, for example, is configured with three cutting edges 175. Thereduced cutting edge height A allows for a smoother cutting, lesschattering, and improved tactile feedback to the user's hand during thebone cutting process. In addition, a ceramic cutting member with fewercutting edges 175 and reduced cutting edge height A can be combined withhigher rotational speeds than prior art metal burrs to cut bone at afaster rate (in terms of grams/min). The system of FIGS. 1 and 2corresponding to the invention operates at up to 20,000 RPM and in onevariation operates at 16,500 RPM for bone cutting. Commerciallyavailable metal burrs typically operate at a maximum of 12,000 RPM. Ifcommercially available metal burrs were operated at higher RPMs, themetal edges would become dull much more rapidly.

Referring to FIGS. 3-4A, a variation of ceramic cutting member 125 hasthree cutting edges 175, however other variations for bone cutting canhave from 2 to 6 cutting edges. In another aspect of the inventionrelating to the cutting edges 175, the reduced number of cutting edgesallows for much higher strength cutting edges in a ceramic body. It hasbeen found that ceramic cutting edges 175 benefit from substantial bulkor thickness B (see FIG. 4A) behind the cutting faces 188 which canprevent a potential fracture in the ceramic, for example, along line 190indicated in FIG. 4A. FIG. 5B shows a prior art metal cutting edge 175′which has relatively little bulk or thickness B′ compared to the ceramiccutting edge thickness B of FIG. 4A. Referring to FIG. 4A, anappropriate manner of characterizing the thickness or bulk of a cuttingedge 175 is to define the cutting edge thickness B as a dimension alonga tangent T to a diameter D at a midpoint MP from a cutting face 188 tothe back side 192 of the cutting edge 175 which is a surface of theadjacent flute 180. As can be seen in the prior art metal cutter of FIG.5B, the prior art thickness B′ of the cutting edge 175′ along tangent T′of diameter D′ at midpoint MP′ of the cutting edge is small in relationto height A′ of the cutting edge due to the ductile, high fractureresistance of metal as opposed to a ceramic (see Chart A above). FIG. 4Billustrates a hypothetical cutting member 125′ that fabricated of aceramic with the cutting edge height A and thickness B of a prior artmetal burr as in FIGS. 5A-5B. In such a ceramic cutter 125′ as depictedin FIG. 4B, the cutting edges 175 would fracture along line 190′ due tothe lack of cutting edge thickness B which equates with strength orfracture resistance. Referring back to the prior art metal burrembodiment of FIG. 5B, the ratio of edge thickness B′ to edge height A′is much less than 1:1. In the cutting member 125 corresponding to theinvention in FIG. 4A, such a ceramic cutting member has a cutting edgethickness B to height A ratio of greater than 1.5, and more oftengreater than 2:1.

In general, as shown for example in FIG. 4A, an arthroscopic cutter 125corresponding to the invention comprises a ceramic body with a pluralityof cutting edges 175 and intermediate flutes 180 wherein each cuttingedge defines a cutting edge or cutting face height A measured from anoutermost peripheral location on cutting edge diameter P to an innermostperipheral location or a flute bottom or surface OS, where the ratio ofthe cutting edge thickness to the cutting edge or face height is atleast 1.5:1 when the cutting edge thickness is measured along a tangentto a midpoint of the cutting face 188 to the adjacent flute. In anothervariation, the ratio of the cutting edge thickness to the cutting faceheight is at least 2:1.

In another aspect, the cutting edge height A relative to the outercutting edge diameter P is small compared to prior art metal burrs suchas illustrated in FIGS. 5A-5B. In the variation of the invention shownin FIGS. 3 and 4A, the cutting edge height A is 0.02 inch which is lessthan 10% of the outer periphery diameter P of the cutting member. Ingeneral, the ratio of the cutting face height A to the peripherydiameter P is 0.2:1 or less, or often such a ratio is 0.1:1 or less.

Another way to define the bulk or thickness of the cutting edges 175 ofceramic cutter 125 (FIGS. 3-4) compared to a prior art metal burr as inFIGS. 5A-5B is to consider the primary relief angle of the cuttingedges. Referring to FIG. 5B, in standard nomenclature for rotarycutters, the primary relief angle E is the angle of the outer surfacejust behind the apex X of the cutting edge 175′. In metal burrs, thereis typically a relief angle of a 2° to 10°, which allows the apex X toengage targeted material even after the apex X becomes dull. It can beeasily understood that as an apex X of a metal cutting edge becomesdull, a relief angle is needed. Otherwise, the rotating cutter couldsimply ride on the backside of the cutting edge 175′ over the targetedtissue. In contrast, turning to FIG. 6, the cutting edges 175 of theceramic cutter 125 of FIGS. 3, 4A and 6 have no primary relief angle atall. Of particular interest, it has been found that since the ceramiccutting edges 175 do not become dull, there is no need (or performancegain) by providing a primary relief angle. Instead, in a ceramic cutter125 corresponding to the invention, the lands 195 have a lands width LWat the outer periphery diameter P than extends over a radial angle ofgreater than 10°, and in the variation of FIGS. 3 and 6, greater than15°. The scope of the invention includes the option of providing someprimary clearance, for example a clearance angle of up to 5°.Alternatively, the amount of clearance can be better defined by the“radial” depth of the clearance, as in a percentage of the peripherydiameter P of the ceramic cutter 125. In general, referring to FIG. 6,an arthroscopic cutter corresponding to the invention comprises aceramic body with a plurality of cutting edges 175 and intermediateflutes 180 wherein each cutting edge 175 has lands 195 with a clearanceof less than 5% of the outer periphery diameter P at a radial angle of15° behind the apex X of the cutting edge 175.

In another aspect of the invention referring to FIG. 4A, the ceramiccutter 125 has cutting edges 175 with a 0° radial rake angle RA whereasmetal burrs always have a substantial positive radial rake angle. Theradial rake angle RA′ of a prior art metal burr of FIG. 5B can rangefrom about 2° to 15°. Positive rake angles are needed in metal burrs orcutters to make such cutters function somewhat effectively as the apex Xof the cutting edge dulls rapidly. Of particular interest, referring toFIG. 4, it has been found that an optimal radial rake angle RA of aceramic cutter 125 is 0°. In other variations, the radial rake angle RAof a ceramic cutter 125 can range from about −5° up to about +10°.

In another aspect of the invention as described above referring to FIGS.3 and 4A, the ceramic cutter 125 has cutting edges 175 that arenon-helical and aligned with the longitudinal axis 115 of the cuttingmember 125. In contrast, typical prior art metal burrs as shown in FIGS.5A-5B have helical cutting edges. This aspect of the ceramic cuttingmember 125 of FIGS. 3-4A that relates to non-helical cutting edgesfacilitates a method of injection molding the ceramic body 125 with athree-component parting mold 200 as shown in FIGS. 7A-7C. FIG. 7A is aschematic sectional view of a parting mold with three parting moldcomponents M1, M2 and M3 and two core pins CP1 and CP2. The mold 200parts along lines 205 and 210 as can be seen in FIGS. 7B and 7C. FIG. 7Bschematically depicts several steps of releasing the green ceramiccutting member 125 from mold 200. Of particular interest, the moldcomponent M1 is adapted to part from the other components M2 and M3 byaxial movement away form distal portion 172 and cutting edges 175 ofceramic cutting member 125 aligned with the longitudinal axis 115 as canbe understood from FIGS. 7A and 7B. It is for this reason that cuttingedges 175 are straight and aligned with the ceramic body's longitudinalaxis 115. In other words, the axially-aligned cutting edges 175 arealigned with the parting direction (the longitudinal axis 115) of moldcomponent M1 (FIG. 7B). As can be understood from the FIGS. 7A-7B, thecutting edges 175 can also have a positive rake angle of up to 5° ormore (see FIG. 4A) and the mold component M1 then can still release fromthe molded green cutting member body 125.

FIG. 7B further shows other steps of the mold release which includeswithdrawal of core pin CP1 in a direction orthogonal to axis 115 toprovide the window 145 in the ceramic cutter 125. This design of themold 200 and core pin CP1 is configured to form the window edges 185Aand 185B with high positive radial rake angles (see FIGS. 3 and 10) aswill be discussed further below. Also, FIG. 7B shows withdrawal of corepin CP2 in the axial direction to provide axial inner channel 126 in theceramic cutting member 125.

FIG. 7C shows another step of the mold release wherein the moldcomponent M2 is moved away from the shaft portion 170 of cutting member125 in a direction orthogonal to the longitudinal axis. Further, themold component M3 is moved relative to the shaft portion 170 of thecutting member to thereby release green cutting member 125 from the mold200. A typical mold 200 will also have ejector pins for pushing thegreen ceramic cutting member 125 from the mold. Such ejector pins arenot shown in the drawings for convenience.

In another aspect of the invention referring to FIGS. 8A-8B, amulti-cavity ceramic injection mold 200′ can be fabricated to mold aplurality of cutting members 125. The multi-cavity mold incorporates themold release parting lines and release directions described above asshown in FIGS. 7A-7C. It can be seen that FIGS. 7A-7B show thesingle-cavity mold 200 in a “side view” relative to the cutting member125 whereas FIGS. 8A-8B show the mold 200′ in a “top view” with respectto the green cutting members 125. FIG. 8A is a sectional view through anexemplary four-cavity mold 200′ although such a mold can have from 2 to16 or more mold cavities. In FIG. 8A, the mold component M2 and core pinCP1 (see FIGS. 7A-7B) are removed so the surface 212 of mold componentM3 is shown with a sectional view of mold component M1. FIG. 8A showscore pins CP2 in sectional view with the cutting member 125 in anelevational top view. FIG. 8B shows how mold component M1 can be movedaxially in alignment with the axis 115 of the cutting members 125 torelease the mold component from the distal portion 172 of a plurality ofceramic cutting members 125 as described previously. Core pins CP2 areshown in a retracted position in FIG. 8B.

In general, an arthroscopic cutting member configured for ceramicinjection molding corresponding to the invention comprises a cuttingmember 125 having a longitudinal axis 115 and a plurality of cuttingedges 175 extending radially outwardly from an outer surface OS, whereinthe cutting member is formed from a wear-resistant ceramic material andwherein each cutting edge is non-helical and aligned with thelongitudinal axis to enable ceramic injection molding with amulti-component parting mold (see FIGS. 3, 4A and 7A-7C).

Now referring to FIGS. 9 and 10, it can be seen that the cutting edgeheight A (FIG. 4A) and the cutting face surface area SA (see hatchedarea in FIGS. 3 and 11) relative to the cutter periphery diameter P issubstantially less than that of the prior art metal burr as shown inFIGS. 5A-5B. As described above, the reduced height A of a ceramiccutting edge 175 (FIG. 4A) when combined with the non-dulling aspect ofthe ceramic edge 175 and the higher rotational speed allows for cuttingbone at a faster rate than prior art metal burrs. As can be understoodintuitively, the cutting edge height A and surface area SA (FIGS. 3 and11) are the key factors that determine the size of bone chips and thecutting rate. In general, referring to FIG. 11, the size of bone chips218 typically is no larger in cross-section than the height A of cuttingedge 175 as the elongated cutting surface does not result in elongatedbone chips. Rather, the cross-sectional dimensions of bone chips 218 areessentially limited to the potential cutting depth (edge height A). Anyelongated cut bone chips will fracture into smaller chips asschematically depicted in FIG. 11. Since the non-dulling ceramic cutter125 cuts bone at a very fast rate, there is a complementary need forfast, efficient bone chip evacuation through the window 145. As outlinedabove, bone chips 218 are evacuated through window 145 into interiorchannel 126 of ceramic cutting member 125 (FIG. 9) and the lumen 128 ofthe inner sleeve 122 (FIG. 11) that communicates with negative pressuresource 160. The bone chips 218 are collected in a collection reservoir220 (see FIG. 2).

In one aspect of the invention, referring to FIGS. 3, 10 and 11, thewidth WW of the window 145 is critically important for the efficientextraction of bone chips, with the window length WL being a suitablelength, for example, at least equal to the window width WW. With thecutting member 125 rotating at 16,500 RPM, it has been found that windowwidth WW is most critical in capturing and then suctioning bone chips218 away from the treatment site. In the variation shown in FIGS. 10 and11, the ratio between the width WW of window 145 relative to the cuttingedge height A is at least 5:1 and often greater than 6:1. This allowsfor bone chips to be rapidly suctioned into and through the window 145and through interior passageway 126 of cutting member 125 in response tothe negative pressure source 160. Further, the diameter C of interiorchannel is large relative to the cutting edge height A (FIG. 10) as willbe described further below.

In another aspect of the invention, referring to FIGS. 10 and 11, thevolume of bone chips 218 resulting from rotation of the cutting member125 is a function of both the height A and length L of the cutting edges175. In other words, the surface area SA of a cutting edge face 188 orfaces and the rotational speed are directly correlated to the cuttingrate in grams/minute of bone removal. It can be easily understood thatit is the cutting edge surface area SA that interfaces with bone andthus cuts a corresponding volume of bone chips. In this regard, thewindow area WA relative to a cutting edge surface area SA is animportant functional metric for a ceramic cutter, and in the variationof FIGS. 9-11, the ratio of the window area WA to a cutting edge surfacearea SA is greater than 8:1. In a typical prior art metal burr as shownin FIG. 5A, the window to edge surface area ratio is much less, forexample about 2:1. In another metric, if the aggregate surface area ofall cutting edges were considered, a ceramic cutter with only 2 or 3cutting edges would have a far higher ratio of window to cutting surfacethan that of a typical metal burr with 6 to 8 or more cutting edges.

Referring to FIGS. 9, 10 and 11, in another aspect of the inventionrelating to extracting bone chips 218 from the treatment site, it can beseen that the diameter C of the interior channel 126 in the ceramiccutting member 125 is substantially larger than height A of the cuttingedge 175. In one variation of FIGS. 9-10, the ratio of the inner channeldiameter C to the cutting edge height A is about 6:1, and the scope ofthe invention includes such a ratio being at least 2:1, at least 4:1 orat least 6:1. In general, the cutting member comprises a wear resistantceramic body carried by an elongate shaft, wherein the ceramic body hasa plurality of cutting edges and flutes intermediate the cutting edges,and a window 145 in the cutting member open to an interior channel 126that communicates with a lumen 128 in the shaft 110 wherein the ratio ofthe diameter C of the interior channel to the height A of the cuttingfaces is at least 2:1. In this variation, each cutting edge 175 definesa cutting edge height A or face height measured from an outer cuttingedge periphery diameter P to a flute bottom diameter or outer surfaceOS.

Referring to FIGS. 3, 9 and 10, in another aspect of the invention, thediameter C of the interior channel 125 of the cutting member 125 islarge relative to the outer periphery diameter P. The ratio of theinterior channel diameter C to the outer periphery diameter P of thecutting edges 125 is at least 0.4:1. In the variation of FIGS. 3, 9 and10, the ratio is 0.048:1. In general, a ceramic cutter differs from ametal burr in that the height A of the cutting edges is small relativeto the outer periphery diameter P and the interior channel diameter Cfor extracting bone chips is large relative to the outer periphery P.Thus, in one aspect of the invention, a cutting member has alongitudinal axis and a plurality of cutting edges extending radiallyoutwardly from an outer surface OS thereof, a window 145 through theouter surface OS communicating with a longitudinal interior channel 126therein, wherein the ratio of the outer surface diameter OS to the outerperiphery diameter P of the cutting edges 125 is at least 0.75:1, andwherein the ratio of the channel diameter C to the outer diameter P ofthe cutting edges 125 is at least 0.4:1.

In another aspect of the invention, referring to FIGS. 12A-12B, thewindow 145 is configured to assist in the extraction of bone chips 218during high-speed rotation in the cut-out 144 region of outer sleeve120. As can be seen in FIG. 10, each window edge 185A, 185B has a sharpapex 225 and more importantly has a radial window rake angle WRA isnon-zero and positive, (see FIG. 10) and typically ranges from about 15°to 45° to capture bone chips 218 as the shaft portion 170 and window 145rotate. In FIGS. 12A-12B, the high radial rake angle WRA (see FIG. 10)of the window edge 185A and apex 225 are shown in assisting in thecapture bone chips 218 in window 145 under the negative pressure in thewindow 145 provided by the negative pressure source 160. In FIG. 12B,the sectional view schematically depicts that the outer edge or apex 225of the window 145 can strike and deflect bone chips 218 inwardly intothe interior channel 126. In contrast, FIGS. 13A-13B illustrates asectional view of the metal burr window 145′ of FIG. 5A under high-speedrotation. The prior art metal burr of FIG. 5A does not have a positivewindow radial rake angle, and in fact has a negative radial rate angleWRA′ (FIG. 13A), so that bone chips 218 are struck by the window face230 instead of a sharp outer edge with a positive rake angle as in theceramic cutter 125 of FIGS. 12A-12B. In the prior art metal burr ofFIGS. 5A and 13A-13B, the substantially negative radial rake angle ofwindow face 230 is fabricated simply by grinding flat faces on the metalsleeve 235 in which the window 230 is formed.

In another aspect of the invention, the ceramic cutting member 125 ofFIGS. 3, 9, 10 and 11 has the distal edge of the window 145 positionedvery close to the proximal end of the cutting edges 175, for exampleless than 0.10 inch or less than 0.05 inch. In prior metal burrs such asin the burr 186 of FIG. 5A, the aspiration window 145′ is necessarilypositioned axially away from the cutting edges 175′ since the metalsleeve 235 need a distal portion configured for welding to the portioncarrying the cutting edges.

Chart B below describes the various dimensions and ratios of the ceramiccutter 125 of FIGS. 3, 4A, 9 and 10 that were described above. This isone variation of a ceramic cutter 125 that has been tested extensivelyand operated at 16,500 RPM to cut bone.

CHART B A Cutting Face Height 0.0205 inch B Cutting Edge Thickness0.0450 inch P Periphery Diameter (Outer Diameter) 0.2590 inch OS OuterSurface Diameter (Shaft Portion) 0.2180 inch C Interior Channel Diameter0.1250 inch L Cutting Edge Length 0.1378 inch SA Cutting Face SurfaceArea 0.0031 sq. in. PA Periphery Surface Area 0.1400 sq. in. RA RadialRake Angle-Cutting Face  0° WRA Radial Rake Angle-Window Edges 35° WWWindow Width 0.1250 inch WL Window Length 0.2062 inch WA Window Area0.0247 sq. in. Ratio- Cutting Face Height to Periphery Diameter 0.08:1Ratio- Window Width to Cutting Face Height  6.1:1 Ratio- Window Area toCutting Face Surface Area  8.1:1 Ratio- Cutting Face Surface Area to P0.01:1 Ratio- Interior Channel Diameter to Cutting Face  6.1:1 Height

As described above with reference to FIGS. 8A-8B, the three componentceramic injection mold 200 with multiple cavities can be used tofabricate the green ceramic cutting members 125 which then after beingreleased from the mold can be sintered to provide the final product. Inother variations of a ceramic injection mold, FIGS. 14A-15C show twocomponent injection molds that can be used to mold the ceramic cuttingmember that can have from 2 to 4 non-helical cutting edges. FIGS.14A-14B first illustrate a two component mold 240 that is configured tomold the ceramic cutting member 245 that has two cutting edges 175. Thecutting member 245 of FIG. 14A is very similar to the cutting member 125of FIGS. 7A-7C above except for the number of cutting edges. As can beseen in FIG. 14A, the mold parting line 248 is on the centerline of thecutting member 245 so that each half of the mold (250A and 250B) canfrom both the shaft portion and distal cutting portion of the cuttingmember (cf. FIGS. 7A-7C). In this variation, there are no undercuts inthe mold 240 so that a simple parting mold is possible. The core pinsfor the window and interior channel can be identical to those shown inFIGS. 7A-7C. An ejector pin for ejecting the green cutting member 245from the mold 240 can be provided, but is not shown for convenience.

FIGS. 15A-15C show another two component injection mold 280 that can beused to mold a ceramic cutting member 285 with 3 cutting edges 175 thatis virtually identical to the 3-edge cutting member 125 of FIGS. 3, 4Aand 7A-7C above. This variation thus illustrates that a 3-edge cuttingmember 285 can be made with a simple two component (one parting line)mold rather than the more complex three component (two parting lines)mold of FIGS. 7A-7B. As can be seen in FIG. 15A, it is necessary toconfigure the cutting member 285 with flat surfaces 288 a and 288 b sothat there are no undercuts in the mold 280. The parting line 290 thencan be off-center. In FIGS. 15A-15B, it can be understood that the sidesof the shaft portion with such flat surfaces 288 a and 288 b allow anupper mold component 295A to be released vertically as shown in FIG. 15Bwhich would not be possible if the outer surface OS was not configuredwith the flat surfaces 288 a and 288 b. As shown in FIG. 15C, thisvariation of mold 280 allows the cutting member 285 having cutting edges175 with zero radial rake angle to be released from the lower moldcomponent 295B with a vertical and slight rotational movement indicatedby the arrows. An ejector pin (not shown) for ejecting the green cuttingmember from the mold can be provided at an appropriate angle relative tothe parting line 290 to push the green cutting member from the mold.This mold embodiment 280 can have core pins CP1 and CP2 as described inFIGS. 7A-7C to form the window 145 and the interior channel 126. It canbe further understood from FIGS. 14A-15B that a two component partingmold with a parting line on the center of the cutting member can be usedto mold a four-edge cutting member.

As described above, several variations of ceramic cutter 125 havenon-helical cutting edges. The non-helical edges allow for simplifiedceramic injection molding. In another variation, a different type ofinjection mold 300 shown in FIGS. 16A-16B can be fabricated to allow formolding a cutting member 305 with helical cutting edges 310 and helicalflutes 312. FIG. 16A shows an injection mold 300 with three componentsthat is similar to that of FIGS. 8A-8B. In this embodiment, the firstand second mold components 315A and 315B are adapted to part asdescribed previously around shaft portion 316 of the cutting members305. The third mold components indicated at 320A-320D are adapted torelease from the green ceramic cutting members 305 by moving axially androtationally (see FIG. 16B). In other words, the mold components320A-320D are moved helically or effectively unscrewed from the cuttingmembers 305. This mold 300 has core pins CP1 and CP2 as describedpreviously to form the window 145 and the interior channel 126 in thecutting members.

In general a method of the invention for fabricating a surgical cuttingmember of a ceramic material, comprises (i) providing an injection moldwith a mold cavity defining outer surfaces of a cutting member having alongitudinal axis, a distal cutting portion with cutting edges, aproximal shaft portion with a window that opens to an interior channelin the ceramic member, (ii) injecting a flowable material comprising aceramic into the mold cavity to provide a molded ceramic member, (iii)removing a first core pin which is configured to form the window, (iv)removing a second core pin which is configured to form the interiorchannel and (v) parting at least first and second mold components thatdefine the outer surfaces of the cutting member to there by release thegreen cutting member from the mold. The method of fabrication furthercomprises sintering the released cutting member to provide a hardenedcutting member.

In the method of fabrication described above, the first core pin isremoved in a direction orthogonal to said longitudinal axis to form thewindow and the second core pin is removed in a direction aligned withsaid longitudinal axis to form the interior channel. Of particularinterest, the core pin that forms the window is configured to provideswindow edges that have a sharp apex 225 and have a high positive windowradial rake angle WRA, for example greater than 15°. Typically, thewindow radial rake angle in the range of 15° to 45°, and extends fromthe outer surface OS to the open diameter C of the interior channel 128,which dimension in one variation can be determined from Chart B above.

In the method of fabrication described above, one mold component ismoved in a direction relative to the cutter body that is orthogonal tosaid longitudinal axis to release the cutter body. In a variation,another mold component may be moved in a direction relative to thecutter body that is aligned with said longitudinal axis thereof torelease the cutting member body. In another variation, a mold componentmay be moved in a direction relative to the cutting member body that ishelical to release the cutting member.

A further method of fabricating the cutting member includes the moldcavity forming a proximal shaft portion having a diameter rangingbetween 0.150 inch and 0.50 inch. Another method of fabrication includesthe mold cavity forming a distal cutting portion having an outerdiameter ranging between 0.10 inch and 0.60 inch. Another method offabrication includes the mold cavity forming non-helical cutting edges.Another method of fabrication includes the mold cavity forming cuttingedges aligned with the longitudinal axis of the cutting member. Anothermethod of fabrication includes the mold cavity forming cutting edgeswith a radial rake angle ranging between 0° and 5°. Another method offabrication includes the mold cavity forming cutting edges having alength ranging 0.10 inch to 0.40 inch. Another method of fabricationincludes a core pin forming the window with an area ranging from 0.01sq. in. to 0.10 sq. in. Another method of fabrication includes a corepin forming the interior channel with a mean cross section ranging from0.008 inch to 0.40 inch.

FIG. 17 is a schematic cross-sectional view of another mold 350 with twoparting components 352A and 352B to form cutting member 354 and furthershowing a core pin 355 partially removed from the upper mold component352B. The core pin 355 is configured with non-parallel side portions358A and 358 b that can be used to form window edges with a range ofpositive window radial rake angles WRA depending on the angle of theside portions 358 a and 358 b.

FIG. 18 is a schematic view of another mold 360 with two partingcomponents 362A and 362B that shows a core pin 365 that extends throughthe ceramic cutting member 366 to provide windows 368A and 368B in bothsides of the cutting member.

FIG. 19 is a longitudinal sectional view of another mold 370 with twoparting components 372A and 372B configured to form cutting member 374.In this a core pin is provided for forming a window 376 that is angledlongitudinally relative to the axis 115 of the cutting member. Core pin378 is configured for forming the interior channel in the cuttingmember.

FIG. 20 is a schematic top view of another mold 380 similar to thosedescribed above showing a ceramic cutting member 385 with a window 386having non-parallel sides 388 a and 388 b which can be formed by asimilarly shaped core pin. In this variation, the angled cutting edges388 a and 388 b provide the advantage of shearing soft tissue capturedin the window in a scissor-like manner as the angled window edges 388 aand 338 b progressively sweep past the lateral edges of the cut-out 144in the outer sleeve 122 (see FIG. 1).

FIGS. 21 and 22 show another variation of ceramic cutting member 400that is intentionally designed with rotational weight asymmetry in thedistal cutting portion 402 thereof. As can be understood from FIG. 21,the proximal shaft portion 405 of the cutter is asymmetric incross-section due to the window 406 and thus does not have a rotationalweight symmetry. At high-speed rotation, for example 16,500 RPM or more,the weight asymmetry may cause a slight vibration or wobbling sensationin the handle by the operator's hand. To overcome the weight asymmetryin the proximal shaft portion 405, the variation of FIG. 21 isconfigured with counter-balancing weight asymmetry in the distal cuttingportion 402. In one variation, the core pin CP1 as shown in FIG. 8A canbe used to provide an off-center void 412 in the interior channel 415within the distal cutting portion 402 as can be seen in FIG. 21. Inanother variation of cutting member 400′ shown in FIG. 22, the interiorchannel 422 can be off center through the proximal shaft portion 424 andthe distal cutting portion to balance the cutting member 400′ relativeto the central axis 115 of the overall cutting member.

Alternatively, another variation (not shown) can have a concavity or inthe exterior surface, such as deeper flutes, in the ceramic body toprovide the weight asymmetry in distal cutting portion tocounter-balance the weight asymmetry in the proximal shaft portioncaused by the window. In another variation (not shown), the cuttingedges can be formed in various asymmetric radial positions to providethe desired weight asymmetry or the cutting edges thicknesses can varyto provide the desired weight asymmetry. In another variation, more thanone of the features described above may be used to achieve the targetedweight asymmetry.

Although particular embodiments of the present invention have beendescribed above in detail, it will be understood that this descriptionis merely for purposes of illustration and the above description of theinvention is not exhaustive. Specific features of the invention areshown in some drawings and not in others, and this is for convenienceonly and any feature may be combined with another in accordance with theinvention. A number of variations and alternatives will be apparent toone having ordinary skills in the art. Such alternatives and variationsare intended to be included within the scope of the claims. Particularfeatures that are presented in dependent claims can be combined and fallwithin the scope of the invention. The invention also encompassesembodiments as if dependent claims were alternatively written in amultiple dependent claim format with reference to other independentclaims.

Other variations are within the spirit of the present invention. Thus,while the invention is susceptible to various modifications andalternative constructions, certain illustrated embodiments thereof areshown in the drawings and have been described above in detail. It shouldbe understood, however, that there is no intention to limit theinvention to the specific form or forms disclosed, but on the contrary,the intention is to cover all modifications, alternative constructions,and equivalents falling within the spirit and scope of the invention, asdefined in the appended claims.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. The term “connected” is to beconstrued as partly or wholly contained within, attached to, or joinedtogether, even if there is something intervening. Recitation of rangesof values herein are merely intended to serve as a shorthand method ofreferring individually to each separate value falling within the range,unless otherwise indicated herein, and each separate value isincorporated into the specification as if it were individually recitedherein. All methods described herein can be performed in any suitableorder unless otherwise indicated herein or otherwise clearlycontradicted by context. The use of any and all examples, or exemplarylanguage (e.g., “such as”) provided herein, is intended merely to betterilluminate embodiments of the invention and does not pose a limitationon the scope of the invention unless otherwise claimed. No language inthe specification should be construed as indicating any non-claimedelement as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

What is claimed is:
 1. An arthroscopic cutter, comprising: a ceramiccutter body having a longitudinal axis and a plurality of cutting edgesseparated by flutes on an outer surface thereof; wherein each cuttingedge has a cutting face with a radial cutting face height measured froman innermost peripheral location on a flute to an outermost peripherallocation on an adjacent cutting face and a cutting edge thicknessmeasured along a tangent to a midpoint of the cutting face to theadjacent flute; wherein each cutting edge has an outer edge diametermeasured from the longitudinal axis of the ceramic cutter body to anouter periphery of the cutting edge and a land with a constant diameterequal to the outer cutting edge diameter, wherein said constant diameterof the land extends over a radial angle of 15° behind the cutting edge;and wherein the ratio of the cutting edge thickness to the cutting faceheight is at least 1.5:1.
 2. The arthroscopic cutter of claim 1 whereinthe ratio of the cutting edge thickness to the cutting face height is atleast 2:1.
 3. The arthroscopic cutter of claim 1 wherein each cuttingedge is non-helical and aligned with the longitudinal axis.
 4. Thearthroscopic cutter of claim 1 wherein least some of the cutting edgeshave a linear cutting edge.
 5. The arthroscopic cutter of claim 1wherein at least some of the cutting edges have a substantially flatcutting face.
 6. The arthroscopic cutter of claim 1 wherein the cutterbody has an outer diameter through the longitudinal axis at theoutermost periphery of the cutting edges wherein a ratio of the cuttingface height to the outer diameter of the cutter body is 0.2:1 or less.7. The arthroscopic cutter of claim 6 wherein a ratio of the cuttingface height to the outer diameter of the cutter body is 0.1:1 or less.8. The arthroscopic cutter of claim 1 wherein each cutting face has aradial rake angle ranging between −5° and +10°.
 9. The arthroscopiccutter of claim 1 wherein the cutter body has an interior channel and awindow with a width and length, the window being open to the interiorchannel therein and configured to communicate with a passageway in ashaft that carries the cutter body, wherein the ratio of window width tothe cutting face height is at least 5:1.
 10. The arthroscopic cutter ofclaim 9 wherein each cutting face has a surface area and the window hasan area that is open to the interior channel; and wherein the ratio ofwindow area to the cutting face surface area is at least 8:1.
 11. Thearthroscopic cutter of claim 9 wherein the interior channel has adiameter and is configured to communicate with the passageway in theshaft; and wherein the ratio of the interior channel diameter to thecutting face height is at least 2:1.
 12. The arthroscopic cutter ofclaim 11 wherein the ratio of the interior channel diameter to thecutting face height is at least 4:1.
 13. The arthroscopic cutter ofclaim 11 wherein the ratio of the interior channel diameter to thecutting face height is at least 6:1.
 14. The arthroscopic cutter ofclaim 9 wherein the cutter body has a distal portion comprising thecutting edges and a proximal shaft portion having the window; andwherein the window has lateral edges with a non-zero positive radialrake angle.
 15. The arthroscopic cutter of claim 14 wherein said lateraledges have a radial rake angle ranging from 15° to 45°.
 16. Thearthroscopic cutter of claim 14 wherein a distal end of the window isaxially spaced apart from a proximal end of the cutting edges by lessthan 0.10 inch.
 17. The arthroscopic cutter of claim 14 wherein a distalend of the window is axially spaced apart from a proximal end of thecutting edges by less than 0.05 inch.
 18. An arthroscopic cutterconfigured to be fabricated by ceramic molding, said arthroscopic cuttercomprising: a cutter body having a longitudinal axis and a plurality ofcutting edges separated by flutes, the cutting edges extending radiallyoutwardly from an outer surface thereof; wherein the cutter body isformed from a wear-resistant ceramic material; wherein each cutting edgehas an outer edge diameter measured from the longitudinal axis of theceramic cutter body to an outer periphery of the cutting edge and a landwith a constant diameter equal to of the outer cutting edge diameter,wherein said constant diameter of said land extends over a radial angleof 15° behind the cutting edge; and wherein each cutting edge isnon-helical and aligned with the longitudinal axis to facilitate ceramicinjection molding of the cutter body.
 19. The arthroscopic cutter ofclaim 18 wherein each cutting edge has a cutting face with a radialcutting face height measured from an innermost peripheral location on aflute to an outermost peripheral location on an adjacent cutting faceand a cutting edge thickness measured along a tangent to a midpoint ofthe cutting face to the adjacent flute; and wherein the ratio of thecutting edge thickness to the cutting face height is at least 1.5:1. 20.The arthroscopic cutter of claim 19 wherein the ratio of the cuttingedge thickness to the cutting face height is at least 2:1.
 21. Thearthroscopic cutter of claim 19 wherein the cutter body has a distalportion comprising the cutting edges and a proximal shaft portion havinga window open to the interior channel therein; and wherein the windowhas lateral edges with a non-zero positive radial rake angle.
 22. Thearthroscopic cutter of claim 21 wherein said lateral edges have a radialrake angle ranging from 15° to 45°.
 23. The arthroscopic cutter of claim21 wherein the window and interior channel are configured to communicatewith a passageway in a shaft that carries the cutter body, wherein theratio of the window width to the cutting face height is at least 5:1.24. The arthroscopic cutter of claim 21 wherein each cutting face has asurface area and the window has an area that is open to the interiorchannel; and wherein the ratio of window area to the cutting facesurface area is at least 8:1.
 25. The arthroscopic cutter of claim 21wherein the interior channel has a diameter and is configured tocommunicate with the passageway in the shaft; and wherein the ratio ofthe interior channel diameter to the cutting face height is at least2:1.
 26. The arthroscopic cutter of claim 21 wherein the ratio of theinterior channel diameter to the cutting face height is at least 4:1.